North Africa or Northern Africa is the northernmost region of Africa. Geopolitically, the United Nations definition of Northern Africa includes eight countries or territories; Algeria, Egypt, Libya, Mali, Morocco, Sudan, Tunisia. Algeria, Morocco, Tunisia, Libya and often Mauritania and Western Sahara are the Maghreb, while Egypt and Sudan comprise the Nile Valley (so named after the Nile River, which has two tributaries; the White Nile and Blue Nile). Egypt is a transcontinental country because of the Sinai Peninsula, which geographically lies in West Asia. North Africa also includes a number of Spanish possessions (Ceuta and Melilla and tiny Spanish islets off the coast of Morocco). The Canary Islands and the Portuguese Madeira Islands, in the North Atlantic Ocean northwest of the African mainland, are sometimes included in considerations of the region. The distinction between North Africa and much of Sub-Saharan Africa is historically and ecologically significant because of the effective barrier created by the Sahara Desert for much of modern history. From 3500 BCE, following the abrupt desertification of the Sahara due to gradual changes in the Earth’s orbit, this barrier has culturally separated the North from the rest of the continent. As the seafaring civilizations of the Phoenicians, Greeks, Romans, Muslims and others facilitated communication and migration across the Mediterranean Sea, the cultures of North Africa became much more closely tied to Southwestern Asia and Europe than Sub-Saharan Africa. The Islamic influence in the area is also significant, and North Africa is a major part of the Islamic world. Some researchers have postulated that North Africa rather than East Africa served as the exit point for the modern humans who first trekked out of the continent in the Out of Africa migration.